Sharp-cutoff pentode
The 4JC6 is a high-performance, miniature 9-pin (Noval) frame-grid sharp-cutoff pentode constructed in a T5½ glass envelope, structurally engineered primarily for high-gain, wideband intermediate-frequency (IF) amplifier stages. Functionally identical to the 6JC6/6JC6A except for its filament parameters, it operates on a controlled series-string heater supply requiring 4.5 V AC/DC at a bogey current of 450 mA, featuring an engineered 11-second warm-up time. In typical operating conditions with a plate voltage (\(E_{b}\)) and screen grid (\(G_{2}\)) voltage of 125 V, it yields a remarkably high transconductance (\(gm\)) of approximately 16,000 to 18,000 micromhos (\(\mu\mho\)), driving high voltage gain even at low signal thresholds. The frame-grid construction minimises interelectrode spacing to achieve ultra-low parasitics, featuring a direct grid-to-plate feedthrough capacitance (\(C_{gp}\)) maxing out at a mere 0.019 pF without external shielding. While originally targeted at RF/IF signal processing, modern audio designers leverage its extreme transconductance, sharp-cutoff behavior, and unipotential coated cathode in specialized preamplifier input stages and hybrid Hi-Fi topologies to achieve wide dynamic range and highly responsive, ultra-low-distortion voltage amplification
